Compositions containing natural or synthetic rubber dispersed or dissolved in a volatile solvent or in an aqueous medium have long been used in the manufacture of "flowed in" gaskets for container closures. In preparing these gaskets, a band of the fluid composition is deposited by means of a nozzle into the sealing area of the closure, i.e. the area which will ultimately engage the body of the container to achieve air-tight and liquid-tight closing. Current and foreseeable future government regulations against solvent emission have rekindled interest in the improvement of the long known and widely used water-based closure sealing compounds so that they not only can be made to perform better in their conventional applications, but also can serve to replace organic solvent-based compounds in as many sealing uses as possible.
One of the features of films or gaskets made from water-based sealing compounds is that said films or gaskets are relatively porous as opposed to films or gaskets obtained from organic solvent-based compositions. In certain applications, this porosity can cause sealing problems because of its generally adverse effects on such gasket properties as adhesion, water uptake and water retention. Porous gaskets contain many paths for water penetration and extraction of whatever water-soluble material is present. This undesirable porosity can be attributed partly to the manner in which the gasket compositions components are mixed together, said manner or process leading to preparations in which each and every component is suspended in water as discrete individual particles. Since most compositions contain large quantities of hard resins and fillers, what happens upon drying is that while the various particles come closer together as the water departs, the polymer and resin particles which theoretically could coalesce into a continuous film are prevented from doing so by the presence of the filler particles. In highly filled systems, there is not enough plastic material to coat every filler particle so that the film obtained consists of islands of resin and polymer with partially coated filler dispersed throughout. Incompletely consolidated films of that type will possess, at the film-substrate interface, large areas of substrate in direct contact with filler. No adhesion is possible at such locations.
The sealing compounds which yield such unconsolidated films are conventionally prepared by adding each component in turn to an aqueous medium as a solid or as an aqueous dispersion, to produce, as mentioned earlier, a liquid in which the polymers, the resins, the fillers and so on are suspended as individual particles.
The principal object of this invention will be to devise a process of forming aqueous sealing compositions which yield films that resemble those obtained from organic solvent-based materials in that the continuity of said films is not severely interrupted by non-adherent hydrophilic pigment and filler particles.